The present invention relates generally to treatment of eye diseases and more particularly to the treatment of eye diseases, such as glaucoma, with hydrogen sulfide (H2S) containing solution or a salt solution capable or generating hydrogen sulfide in situ.
Glaucoma is a blinding disease of the eye that affects approximately three million people in the United States, with more than 120,000 blind due to the condition. Glaucoma is the 2nd-leading cause of blindness in the United States. Elsewhere in the world, glaucoma treatment is less available, and glaucoma ranks as a leading cause of blindness just about everywhere. Even if people with glaucoma do not become blind, vision can be severely impaired. When the pressure in the eye (intraocular pressure—IOP) increases to dangerous levels, it damages the optic nerve. This can result in decreased peripheral vision and, eventually, blindness. Glaucoma is similar to ocular hypertension but with accompanying optic nerve damage and vision loss.
Several attempts have been made to decrease the intraocular pressure in the eye as a treatment for glaucoma. One method (the present invention) uses an unlikely substance—hydrogen sulfur (H2S).
H2S is a colorless gas with a pungent odor that is produced endogenously in mammalian tissues from L-cysteine. H2S is generated from L-cysteine in a reaction catalyzed by two pyridoxal-5′-phosphate dependent-enzymes, cystathionine β-synthase (CBS; EC 4.2.1.22) and cystathionine γ-lyase (CSE; EC 4.4.1.1). Both CBS and CSE are enzymes of the trans-sulfuration pathway that inter-converts L-methionine and L-cysteine but can also use L-cysteine as an alternative substrate to form H2S. Both CBS and CSE are needed for the production of H2S in tissues. The expression of CBS and/or CSE is dependent on the tissue under study. For instance, several mammalian cells including those from the brain, liver, kidney, skin and blood have been reported to possess CBS as well as CSE activities.
No activity or expression of CBS was detectable when testing human atrium and ventricles, internal mammary arteries and saphenous veins indicating that this enzyme may not play a major role in generating H2S in these tissues under physiological conditions.
In contrast, CSE is the main H2S generation enzyme that has been identified, cloned and expressed in the rat vascular bed and heart. In 1995, Watanabe and co-workers (Proc. Natl. Acad. Sci. USA, 92: 1585-1589) produced a CBS knock-out mouse model that represented the disease, homocysteinemia. Using CBS knock-out mice, Eto and coworkers (2002; Biochem. Biophys. Res. Commun. 293: 1485-1488) also showed evidence that CBS produced the endogenous H2S in the brain.
In 1989, Goodwin and colleagues (J. Anal. Toxicol. 13: 105-109) developed a highly sensitive method for the measurement of H2S concentrations in tissues. With the highly sensitive and accurate methodology, the concentration of H2S has been determined in some biological tissues and fluids. For instance, the physiological concentration of H2S in brain tissue has been shown to be in the range of 50 μM to 160 μM. In rat and human blood, concentrations of H2S have been reported to be 50 μM and 10-100 μM.
Because of its high lipid solubility, H2S easily diffuses across biological membranes. When oxidized, H2S yields sulfur oxide, elemental sulfur and sulfates and can be hydrolyzed to form a hydrosulfide anion and sulfide ion. Approximately 30% of H2S exists as the undissociated form while about 60% is in the dissociated form as the hydrosulfide anion at a pH of 7.4.
In the environment, H2S is the predominant sulfur contaminant of natural gas and is a by-product of sewage treatment and paper pulp mills. H2S is a component of foodstuffs such as dairy products, cooked meats and human feces and is a product of bacterial and helminth metabolism. The toxic effect of acute or chronic exposure of vertebrates and invertebrates to H2S is well known.
In the eye, toxicity associated with exposure to lethal concentrations of H2S is mostly at the mucus membrane level leading to keratoconjunctivitis. Until recently, most studies of the biological actions of this gas have focused on its toxicity. Because there is evidence for in situ formation of H2S by mammalian cells, attention has now been focused on the potential physiological role of this gas. Based on the relatively high concentrations of H2S in the brain, this gas has been hypothesized to play a physiological role in the brain and indeed, could be involved in some of the diseases of the central nervous system.
For example, the observation that physiological concentrations of H2S enhance the activity of NMDA receptors and alter the induction of long term potentiation in the hippocampus led Abe and Kimura (1996; J. Neurosci. 16: 1066-1071) to conclude that endogenous H2S serves as a neuromodulator in the brain. H2S has also been implicated in some diseases with an overproduction reported in patients with Downs Syndrome. Eto and coworkers (2002; Biochem. Biophys. Res. Commun. 293: 1485-1488) showed that brain H2S is significantly reduced in patients with Alzheimer's disease indicating the role of this gas in the pathophysiology of this disease. Apart from the brain, H2S has been reported to produce pharmacological effects on the cardiovascular system and a direct relaxant action on vascular and non-vascular smooth muscles (reviewed by Moore et al., 2003; TIPS, 24: 609).
Unlike the central nervous and cardiovascular systems, no study has addressed the pharmacological effects of H2S on intraocular pressure, its potential action on the release and/or availability of neurotransmitters in the anterior uvea or on the direct action of this gas on ocular smooth muscle function. Observation of a significant regulatory action of H2S on intraocular pressure or its presence in physiological concentrations in the anterior uvea opens up new opportunities in glaucoma and other eye treatment.
It is, therefore, an object of the present invention to provide a method of reducing interocular pressure in the eye by using H2S.
It is well known that exposure to high concentrations of H2S is lethal in both human and experimental animals. However, by using much lower concentrations, H2S has been reported to produce a wide range of physiological effects in several species. For instance, H2S produced negative inotropic effects in rats in both in vivo and ill vitro experiments, an effect that was blocked by glibenclamide, an KATP channel antagonist. Pharmacological studies of the effect of HIS have utilized sodium sulfide (Na2S) and/or sodium hydrosulfide (NaHS) because of their ability to generate this gas in vivo. There is evidence that at a physiological pH of 7.4, about 30% of sulfide whether derived from gaseous H2S or one of its alkali salts will exist in the form of H2S, with the anion HS− consisting of the balance. For instance, NaHS dissociates to form Na+ and HS− in solution, and then HS− combines with H+ to form H2S. The use of NaHS as a source of H2S has been reported to accurately and reproducibly define the concentration of this gas in solution than bubbling H2S gas. Consequently, both NaHS and Na2S were employed to generate H2S in vivo and in vitro.
One of the most extensively studied biological actions of H2S has been within the central nervous system. Chronic contact of neonatal rats with low concentrations of H2S has been reported to increase serotonin and norepinephrine concentrations in the cerebellum and frontal cortex. Dello Russo and co-workers (2000; J. Endocrinol. 12: 225-233) found that although NaHS had no effect on basal secretion of corticotrophin-releasing hormone (CRH) from rat hypothalamic explants, H2S consistently inhibited KCl-stimulated release of CRH. Physiological concentrations of H2S have also been shown to facilitate hippocampal long term potentiation (LTP) in rats by enhancing the N-methyl-D-aspartate (NMDA)-induced inward current. There is evidence that cyclic AMP may mediate the effect of H2S on NMDA receptors. In summary, evidence from literature confirms an effect of H2S on neuronal function
A review of the literature revealed that most of the pharmacological studies of the postjunctional actions of H2S have been on smooth muscles. For example, H2S has been shown to cause relaxation of pre-contracted rat aortic and uterine smooth muscles, in vitro. In uterine smooth muscle, both L-cysteine and NaHS (used as potential H2S donors) inhibited spontaneous contractions whereas other related amino acids had no such effect. Zhao and coworkers (2001; EMBO J. 20: 6008-6016) found that a small portion of H2S induced relaxation in the aorta was dependent on the presence of the endothelium and nitric oxide (NO) and could be mediated by KATP channels. In gastrointestinal and urogenital smooth muscles, Teague and colleagues (2002; Br. J. Pharmacol. 137: 139-145) showed that NaHS caused a concentration-related relaxation of isolated rabbit ileum and rat vas deferens. NaHS also inhibited the contractile response of the guinea pig and rat ilea preparations to electrical stimulation of intramural nerves. Furthermore, Teague et al. found that effects caused by NaHS were blocked by inhibitors of CSE and CBS confirming that H2S formed from this compound was responsible for these responses.
In rats, an intravenous bolus injection of H2S caused a transient decrease in blood pressure, a response that was blocked by pretreatment of animals with KATP channel antagonists. In a study to establish whether an impaired H2S pathway was associated with hypertension, Zhong and coworkers (2003; J. Hypertension 21: 1879-1885) induced this disease by treating rats with the NO synthase inhibitor, L-NAME and then administered NaHS or vehicle to two groups of animals. Treatment with NaHS effectively prevented the development of hypertension suggesting that a dysfunction of vascular H2S pathway was involved in L-NAME induced hypertension. Geng and colleagues (2004; Biochem. Biophys. 313: 362-364) also reported that an intravenous bolus injection of NaHS produced a decrease in central venous pressure in rats. In spontaneously hypertensive rats, Yan and coworkers (2004; Biochem. Biophys. Res. Commun. 313: 22-27) found that exogenous administration of H2S attenuated the elevation of blood pressure and reduced the structural remodeling of the aorta associated with the development of hypertension. Clearly, the evidence available supports a role for H2S in the pathogenesis of hypertension in rats.
It is, therefore, an object of the present invention to establish a new therapeutic strategy that applies hydrogen sulfide to the eye to cause a lowering of intraocular pressure.